The circuit patterns of semiconductors have been increasingly miniaturized in recent years, and there has been a demand for higher resolving power in exposure apparatuses that print these patterns. To satisfy this demand, the wavelength of the light source must be made shorter and the NA (numerical aperture) of the optical system (i.e., projection lens) must be made larger. Nevertheless, the types of optical glass that can withstand practical use are limited due to the absorption of light as the wavelength shortens. Once the wavelength falls below 180 nm, the only usable glass, for practical purposes, is fluorite.
However, it is impossible to correct chromatic aberration if the optical system is a refractive (i.e., dioptric) optical system with only one type of glass. Accordingly, it is extremely difficult to build a dioptric optical system having the required resolving power. Therefore, a variety of proposals have been made for a catadioptric reduction optical system, i.e., one that combines a reflective system with a refractive system comprising optical glass that can be used at the working wavelength.
Certain of the proposed optical systems form an intermediate image one or more times midway in the optical system. Others form an intermediate image just once, such at those systems disclosed in Japanese Patent Application Kokoku No. Hei 5-25170, Japanese Patent Application Kokai No. Sho 63-163319, Japanese Patent Application Kokai No. Hei 4-234722 and U.S. Pat. No. 4,779,966.
Among the abovementioned prior art, the optical systems having just one concave mirror include those disclosed in Japanese Patent Application Kokai No. Hei 4-234722 and U.S. Pat. No. 4,779,966. These optical systems employ only negative lenses in the round-trip combination optical system which includes the concave mirror, and do not use optical systems having positive power. Consequently, since the light beam widens as it travels toward the concave mirror, there is a tendency for the diameter of the concave mirror to increase.
Japanese Patent Application Kokai No. Hei 4-234722 in particular discloses a round-trip combination optical system that is completely symmetrical. The generation of aberrations in this system is maximally controlled, making the correcting aberrations in successive refractive optical systems easier. However, since the system is symmetric, it is difficult to obtain sufficient working distance in the vicinity of the object plane, thereby requiring the use of a half-prism.
The optical system disclosed in U.S. Pat. No. 4,779,966 includes first and second imaging optical systems, and uses a mirror in the second imaging optical system, rearward of the intermediate image. However, to ensure adequate brightness, the light beam needs to widen as it approaches the concave mirror. Thus, it is difficult to make the concave mirror compact.
There is a possibility, with certain types of optical systems employing a plurality of mirrors, that the number of lenses in the refractive optical system can be reduced. However, these types of systems have a number of shortcomings. For example, a phase shift method has been conceived recently that shifts the phase of selected portions of the mask, thereby raising the resolving power while preserving the depth of focus. Further, the ratio .sigma. between the NA of the illumination optical system and the NA of the imaging optical system has been made variable to enhance the imaging performance. Although an aperture stop can be installed in the illumination optical system to vary .sigma., an effective installation location for the stop cannot be found if the catadioptric optical system mentioned earlier is made the objective lens.
In catadioptric optical systems of the type employing a round-trip optical system of such an arrangement on the reduction side, a sufficient distance from the reflective mirror to the wafer (image plane) cannot be obtained due to issues related to the reduction magnification. Consequently, it is unavoidable that the brightness of the optical system is limited, since the number of lenses constituting the objective lens that can be inserted in the optical path is limited. For example, even if a high-NA optical system could be realized, a sufficient working distance WD between the wafer and the most wafer-wise surface of the projection lens could not be obtained due to the need to have a large number of optical members in a limited optical path length.
In addition, it is necessary in such conventional catadioptric optical systems to fold the optical path. However, the procedure of adjusting the optical path bending member is difficult, making a high-precision system difficult to realize.
Certain twice-imaging optical systems have many excellent advantages. However, to separate the light beam incident the concave mirror and the light beam reflected from the concave mirror, these optical systems must employ methods, such as using a light beam separating prism or an apertured mirror, or perform separation with a reflective mirror using an off-axis light beam.
With any of these methods, the optical axis of the optical system must be bent, at a right angle for example, using a reflective surface. Although this is quite easily accomplished compared with a conventional catadioptric optical system, this places a heavy burden on the adjustment mechanism of the optical system compared to an optical system based on just a conventional refractive system.
In other words, since most optical systems, including refractive optical systems, are constructed on a single linear optical axis, if the lens is mounted shifted or tilted with respect to this optical axis, the problem can be corrected by rotating the lens about this linear optical axis, and examining the reflected light and the like from the lens. Such an adjustment means cannot be adopted and the adjustment method becomes problematic if the optical axis is bent.
Furthermore, only two types of adjustments exist in an optical system comprising a single optical axis: tilting and shifting from the optical axis. However, with an optical system having a plurality of optical axes, six degrees of freedom arise in the three-dimensional space in a lens shifted from the single optical axis that forms the reference: the positional coordinates X, Y, Z, and the rotational angles .alpha., .beta., .gamma. about the X, Y, Z axes. Also, the number of components requiring such adjustments increases significantly. Consequently, the adjustment time increases, the cost increases, and there are numerous difficulties just in realizing the design performance in a precision optical system. The question of how to eliminate such troublesome adjustment has been a pending and critical issue in round-trip optical systems.